BOSTON (AP) - Federal agents are zeroing in on how the Boston Marathon bombing was carried out, but there's still no word on who did it and why.

An intelligence bulletin issued to law enforcement and released late Tuesday includes a picture of a mangled pressure cooker and a torn black bag. The FBI says evidence indicates the bomber packed a pressure cooker with explosives, nails and ball bearings and hid it in a back pack. The agency says the second bomb was in a metal container, but there isn't enough evidence to indicate that it was a pressure cooker.

The FBI and other law enforcement agencies continue to ask members of the public to come forward with photos, videos or anything suspicious they might have seen or heard.

Many of the more than 170 wounded remain in hospitals, many with serious injuries. A 9-year-old girl and 10-year-old boy are among 17 victims listed in critical condition.

The three people killed include an 8-year-old boy, a 29-year-old woman and a graduate student from China.

The FBI agent in charge says "the range of suspects and motives remains wide open."

(Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)

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